Attachments are different than documents and are only available for the following objects:

Account

Asset

Campaign

Case

Contact

Contract

Custom objects

EmailMessage

EmailTemplate

Event

Lead

Opportunity

Product2

Solution

Task

Salesforce.com restricts an attachment size to a maximum size of 5 MB. For a file attached to a Solution, the limit is 1.5MB. The maximum email attachment size is 3 MB. You can contact Salesforce.com support and possibly have them increase these limits. They should be able to increase the document and attachment size to 25MB. They cannot increase the limits for emails.